There is a class of instruments known as a laser tracker that measures the coordinates of a point by sending a laser beam to a retroreflector target in contact with the point. The instrument determines the coordinates of the point by measuring the distance and the two angles to the target. The distance is measured with a distance-measuring device such as an absolute distance meter or an interferometer. The angles are measured with an angle-measuring device such as an angular encoder. A gimbaled beam-steering mechanism within the instrument directs the laser beam to the point of interest.
The laser tracker is a particular type of coordinate-measuring device that tracks the retroreflector target with one or more laser beams it emits. There is another category of instruments known as total stations or tachymeters that may measure a retroreflector or a point on a diffusely scattering surface. Laser trackers, which typically have accuracies on the order of a thousand of an inch and as good as one or two micrometers under certain circumstances, are usually much more accurate than total stations. The broad definition of laser tracker, which includes total stations, is used throughout this application.
Ordinarily the laser tracker sends a laser beam to a retroreflector target that is typically located on the surface of an object to be measured. A common type of retroreflector target is the spherically mounted retroreflector (SMR), which includes a cube-corner retroreflector embedded within a metal sphere. The cube-corner retroreflector includes three mutually perpendicular mirrors. The vertex, which is the common point of intersection of the three mirrors, is located near the center of the sphere. Because of this placement of the cube corner within the sphere, the perpendicular distance from the vertex to any surface of the object on which the SMR rests remains nearly constant, even as the SMR is rotated. Consequently, the laser tracker can measure the 3D coordinates of a surface by following the position of an SMR as it is moved over the surface. Stated another way, the laser tracker needs to measure only three degrees of freedom (one radial distance and two angles) to fully characterize the 3D coordinates of a surface.
Some laser trackers have the ability to measure six degrees of freedom (DOF), which may include three translations, such as x, y, and z, and three rotations, such as pitch, roll, and yaw. An exemplary six-DOF laser tracker system is described in U.S. Pat. No. 7,800,758 (‘758) to Bridges, et al., incorporated by reference herein. The ‘758 patent discloses a probe that holds a cube corner retroreflector, onto which marks have been placed. A retroreflector onto which such marks have been placed is called a six-DOF retroreflector. The cube corner retroreflector is illuminated by a laser beam from the laser tracker, and the marks on the cube corner retroreflector are captured by a camera within the laser tracker. The three orientational degrees of freedom, for example, the pitch, roll, and yaw angles, are calculated based on the image obtained by the camera. The laser tracker measures a distance and two angles to the vertex of the cube-corner retroreflector. When the distance and two angles, which give three translational degrees of freedom of the vertex, are combined with the three orientational degrees of freedom obtained from the camera image, the position of a probe tip, arranged at a prescribed position relative to the vertex of the cube corner retroreflector, can be found. Such a probe tip may be used, for example, to measure the coordinates of a “hidden” feature that is out of the line of sight of the laser beam from the laser tracker.
One common application of a laser tracker is to measure a relatively large object to see how its actual dimensions compare to the design dimensions (e.g., as given by CAD data). There may be several of these objects utilized in a particular application and the objects are typically expected to be identical in geometry. Any distortion in the geometry of the object either initially or developed over time can influence other operations in the overall system that the object is a part of. For example, if the object is bent or twisted in any way it can lead to manufacturing defects and poor product quality.
Typically as known at least three points are required to establish the relationship between the laser tracker and the object for measurement purposes. As is known in the art, the ability of the operator to manually measure these initial points with sufficient accuracy is an area for consideration.
Thus, there is a need for an operator of a laser tracker or similar measurement device to be able to not have to manually measure the targets points (e.g., SMRs). Instead, it would be desirable for the operator of the laser tracker to utilize the camera system in the laser tracker to automatically measure all of the target points required for any particular application, thereby significantly reducing the possibility of operator error in the measurement process and not requiring specialized skills and/or training.
More generally, there is a need for a method and a system in which the laser tracker automatically carries out many of the functions that would previously have to be carried out manually. It would be desirable to quickly obtain consistent measurements with the laser tracker, even if the measurements are carried out by an unskilled operator. Typical measurements include tool inspection measurements; for example, the carriage in a body-in-white assembly line is an example of a tool to be inspected or monitored. Other examples of tools include a sheet metal stamping jig, and an assembly tool for assembling a portion of an aircraft structure. Generally, for almost every part made in an automotive or aerospace application, there is a corresponding tool. Thus, it would be desirable to improve the process of measuring such tools with a laser tracker. In addition, it would be desirable to apply the measurement process to finished parts as well.
When using one or more cameras on a laser tracker to locate and drive to retroreflector targets in an automated procedure, occasionally an SMR is not found in an expected location. Inability to find a retroreflector may include an SMR not placed in a designated nest, an SMR rotated at a large angle, or an obstruction placed between the tracker and the retroreflector. In one mode of operation, a tracker may lock onto the retroreflector nearest the designated target position. In some instances, this may result in the tracker measuring the wrong retroreflector and hence determining the wrong location. Accordingly, while existing trackers are suitable for their intended purposes a need for improvements remains, particularly in providing an improved method for responding to the inability to lock onto a retroreflector target at an expected location.